This invention relates generally to a liquid dispensing system for use in clinical analyzers. More particularly the invention relates to a liquid dispensing system which includes acoustic sensing means.
Pipettes are employed in automated analyzers for transporting liquids between reservoirs which hold liquid samples and/or reagents and for transporting liquids from such reservoirs to test sites such as in assay cartridges to conduct various tests. The pipette is typically carried by a transport mechanism which provides for both horizontal and vertical movement so as to enable the pipette tip to be lowered into a liquid in a reservoir for aspiration of the liquid, and for transporting the liquid to a test site whereat the pipette is lowered to an optimal position for dispensing the liquid. Some type of device, such as a piston assembly, which may be incorporated into the pipette, is operated electronically to aspirate liquid into the pipette and to dispense liquid from the pipette.
Various types of chemical tests can be performed by such automated test equipment, an example of testing of considerable interest being the assay of biological substances for human health care. Automated test equipment allows large numbers of test samples to be processed rapidly. Such equipment is employed in health care institutions including hospitals and laboratories. Biological fluids, such as whole blood, plasma or serum are tested to find evidence of disease, to monitor therapeutic drug levels, etc. In such automated analyzers disposable pipette tips are typically used for the delivery of one liquid only and then discarded so as to avoid contamination which could lead to errors in the assay result.
It is desirable, when aspirating liquid into the pipette tip, to lower the orifice of the pipette tip into the liquid to a controlled distance which is sufficient for the aspiration of the desired amount of liquid. Inaccurate positioning of the pipette tip relative to the surface of the liquid may introduce an error in the amount of liquid aspirated into the tip. Further, lowering the tip into the liquid to an excessive distance increases the possibility that a small amount of liquid may adhere to the outside wall of the pipette tip when the tip is withdrawn from the liquid. Liquid adhering to the outer wall of the pipette tip could result in an inaccurate amount of fluid being dispensed.
The use of disposable pipette tips presents a problem in controlling the depth to which the pipette tip is lowered into a liquid. The disposable pipette tips, which are typically made of a polymeric material, are provided in a storage tray within the instrument. Initially the pipette, which typically has a metal stem, is advanced downwardly to secure a disposable tip by frictional contact. Since the polymeric materials from which the tips are made are flexible there may be some variation from tip to tip as to the distance of the tip orifice from the metal pipette stem. Thus, any variation in the positioning of the tip on the pipette stem can result in an error in the desired positioning of the pipette tip in the liquid during the aspiration step.
In addition to the above mentioned consideration it is preferable that such automated analyzers, the operation of which is typically controlled by a software program embedded in a microprocessor, be able to recognize whether a disposable pipette tip has in fact been properly attached to the pipette stem during the dispense cycle. Further, it would be advantageous for the analyzers to have the capability to determine the level of liquid aspirated into the pipette tip.
The prior art liquid dispensing devices are not satisfactory in all instances. For example, devices which are capable of sensing the liquid level during the aspiration step may not be able to determine the amount of liquid which is aspirated into the pipette tip. Accordingly, as the state of the art advances and more demands are made on the instrument systems there is a continuing need for novel liquid dispense systems.